Let Me In is an American-British horror film based on the novel Let The Right One In by John Ajvide Lindqvist. It tells the story of a 12-year-old boy in a New Mexico town who develops a friendship with a vampire child in the early 1980s. Artistic rights for the English-language film adaptation were acquired at the 2008 Tribeca Film Festival, where the Swedish version won the "Founders Award for Best Narrative Feature," and Matt Reeves was quickly introduced as the director after Swedish director Tomas Alfredson turned down the project. Reeves has stated that he will "...make a new film based on the book, and not remake the Swedish film..." so that "...it'll be something completely different...", although he later admitted that he used exclusive scenes from the Swedish film "There are things that [Lindqvist] adapted brilliantly in the movie and I borrowed from that because I thought he did a great adaptation" and that he didn't really add anything that different from the novel into his film as "the story was so big that you couldn't really add a lot of stuff in without taking away the focus of the coming of age story".

In 1983 in Los Alamos, New Mexico, an ambulance and police cars are hurriedly driving to a hospital. From the ambulance and into a police-guarded tenth-floor hospital room is a man, suspected for murder, with burns covering 9-9.5% of his body, with his head and neck burned beyond identification. The Policeman enters the room to interrogate the Suspect, yet as the Suspect's mouth and vocal chords are too damaged to produce speech, he gives him a pencil and notepad to communicate with. When the Suspect refuses to write anything, the Policeman suspects him of being in a criminal Satanic cult, and tells him that he will catch whoever else is in league with him. The Policeman is then called out to the reception area by a nurse to take a phone call. While he is on the phone, a nurse enters the Suspect's room, and scream is heard from there. When the detective returns he sees that the Suspect has fallen from the window ten stories to his death. The camera then shows the Suspect's notepad, with a note reading, "I'm sory [sic] Abby."

Two weeks earlier, living in a suburban apartment complex, Owen is a miserable 12-year-old boy, who has no friends, has neglectful, divorcing parents (his father left, his mother an alcoholic), and is mercilessly bullied at his school every day. As Owen is in his room, he dons a mask and wields a kitchen knife, he looks in a mirror and threatens an imaginary foe, while hurling the same insults at it that his bullies do. He then uses a telescope to watch and spy on his neighbors through the windows. As he does so, he notices a girl appearing to be his age with a man who appears to be her "Father" moving in next door. The girl at first tells Owen that she cannot be his friend, not telling him why, but eventually out of loneliness she soon starts to befriend him, and tells him that her name is Abby. Abby and Owen grow closer and closer, hanging out late at night and developing feelings for one another. Abby claims and appears to be 12 years old, but she talks and acts as if she is much older. During school Owen writes down the Morse code on two sheets of paper for him and Abby to use to communicate to each other through their walls. He encounters his bullies (Kenny, Mark, and Donald) in the bathroom between classes, and Kenny scars his face with an antenna rod he carries around with him for refusing to hand over his sheet with the Morse code on it. Owen meets Abby later that night and gives her and explains the system of Morse code. Abby then notices his band aid covering his cut, and when Owen admits being bullied to her, she tells Owen to hit back, and that she will help him if needed. A confused Owen then questions how she can help, saying "but you're a girl," but Abby holds his hand and assures him that she is stronger than she seems. Abby continues growing closer emotionally to Owen by using the Morse code to communicate with him by tapping on the wall from her apartment and as they date at night. We see that Abby rudely tells her "Father" to get out of the living room and leave her alone, so that she can use the wall to send the coded messages to Owen in privacy. The "Father" meekly cooperates and goes to another room, and we see that Abby has considerable power over her "Father" and that she can be demanding.

Meanwhile, Abby's "Father" periodically goes out at night to kill local residents so that he can draw out their blood to bring back to Abby, a centuries-old vampire, for her to feed on, as blood is the only thing she can survive on. His first attempt is successful in getting a victim at a grocery store, and stringing him upside down in the snowy woods, but when he is collecting the blood, his leg falls through thin ice and spills all the blood. In a panic from an oncoming car, he flees the scene, returning home to Abby with nothing. Returning to a furious Abby, he admits to her that he is growing tired of killing for her. The next night, Abby goes outside and tricks a local jogger (Jack) by pretending to be hurt, and as he picks her up, she attacks, bites, and feeds on him for blood. To prevent him from getting up as a vampire, she kills him by snapping his neck after she was finished. When she comes home and tells her "Father," he curses at her, furiously leaving to dispose of Jack's corpse into a nearby lake. One night, the "Father" furiously prepares his tools to kill for Abby that night, and then Abby gently touches his face and arm, seeming to alleviate his troubles inside. When he asks Abby to stop seeing Owen, she says nothing, and he leaves the apartment in a renewed state of fury. That night, the "Father" goes out and picks a car at the local high school where he can break and hide in until the owner returns, planning to kill him for his blood to feed Abby. However, when the owner returns, a friend is with him, which complicates matters. They stop at a gas station and the driver leaves the car to enter the station's corner shop. While the passenger stays in the car, he notices the "Father" in the backseat, and the passenger is attacked and killed. However, when the "Father" attempts to drive away, he crashes and rolls down an embankment on the side of the road. Feering that his identity will be discovered, the "Father" douses his face with concentrated acid so that his face will not be recognized and his connection to Abby will not be discovered by the police. The "Father" is taken a hospital into a tenth-floor police-guarded room, and when Abby hears on the news that he is there, she goes to visit him by climbing up the side of the hospital wall outside his window. When he leans out the window, Abby is stricken with grief over what had happened to him. To end his misery and feed herself, Abby kills her "Father" by biting his neck and sucking out his blood after he implores her to do so, and he falls out the window to his death.

Abby is traumatized and confused by the loss of her "Father." She goes to Owen's bedroom window and while he is asleep, she asks him if she can come in. Owen allows her into his room, and he asks her to go steady and be his girlfriend. She rejects him first, but when Owen tells her that nothing will change between them, she accepts, and the relationship between the Owen and Abby deepens. The Policeman gradually learns of the "Father's" past and eventually his connection to Abby. The next day on a field trip at a frozen lake, Owen stands up to his bullies when confronted, and hits Kenny in the head with a pole, splitting his ear and causing him to have a concussion and be hospitalized. Mr. Zoric, the gym teacher, witnesses this and tries to go to the boys, but before he could, one of the girls screams in fear after discovering the dead body of Jack which the "Father" disposed of earlier. Owen almost got suspended but didn't. When Owen tells Abby about his encounter that night, Abby leans down and kisses him lightly on the cheek. He then takes her down to an abandoned room of the basement of the apartment complex as a surprise. It is said by Owen that the adults are unaware of the room, and that only he, Abby, and his former neighbor Tommy (a reference to Tommy from the Swedish novel) are aware of it. He wants to surprise her and he has her close her eyes, and then he cuts his finger to make a blood pact with her. Abby is taken aback by the blood falling to the ground from his finger and drops to the ground to lick up his blood. She looks up and Owen is able to see her true Vampiric face, not wanting to attack Owen. Abby runs from Owen and attacks a woman in the complex park named Virginia. Abby jumps from a high tree branch, tackles down, and bites Virginia's neck and tries to drink her blood, but she is wrestled off of her and chased away by Virginia's boyfriend, Larry.

Owen comes to terms that Abby is a vampire, and still likes her the same, as she is able to provide the affection he could not find in the mortal world. He goes to her apartment to confront her, he asks her where her "Father" is, but she tells him that he was not her real father. As Owen looks through Abby's belongings, he finds an aging photostrip showing Abby with a young boy who appears to be her deceased guardian as a child, who she befriended (and was possibly in love with) decades ago. At this point, Owen leaves, either out of horror or jealosy, while Abby is repeating as earlier in the film that they could not be friends. At the hospital the next morning, the police detective learns from Larry that the girl who attacked Virginia in the complex matches the description of who he thought was the real daughter (Abby) of the Ritual Killer (The Father), whom he suspected both of being involved in a criminal Satanic cult. While the policeman is questioning Larry outside the room, Virginia, unaware that she is a vampire now, begins to feed on her own blood by biting into her own arm. As a nurse opens the blinds to the room's windows, Virginia becomes ingulfed in fire on exposure to the sunlight, immolating both her and the nurse to death.

That night, Abby visits Owen's apartment while his mother is away, and asks if she can come in. Owen opens the door wider, indicating that Abby can come in, but then Abby says that he has to invite her in verbally. A curious Owen refuses to do so, wondereing what were to happen if Abby were to enter a private residence without verbal invitation, and pressures her to do so. As she does, she begins to hemorrhage out of all of her bodily orifices and pours in her skin, only to stop when Owen screams that she can come in. They ebrace each other, and he asks if she would have bled to death if he did not verbally invite her in, and she responds with, "I knew you wouldn't let me." Owen cleans up the blood while Abby is showering, and as she leaves the bathroom while wearing only a towel, Owen directs her to his mother's room where he says she can change into one of his mother's old dresses. Then, while she is changing, Owen sneaks a look at her in through a crack in the door, and is seen with a facial expression of shock, leaving Abby's gender questionable, or at least so to those familiar with the Swedish story. Once Owen's mother suddeny arrives home, she was able to escape from Owen's apartment through the window before his mother saw her. While Owen's mother drank herself to sleep on the sofa, he sneaks from his apartment to Abby's and spends the night there.

The next day, the detective searches for Abby and traces her to her apartment. He initially knocks on the door, but while Owen tries sneaking to the door lens to see the Policeman, he accidently makes the floorboards creak under his foot. The Policeman hears Owen, and breaks down the door and enters the apartment, but not before Owen successfully ducks and hides behind a wall. The Policeman finds a note written by Abby to Owen that she is sleeping in the bathroom, and finds her in there. When he tries tearing away the cardboard layer covering the window to let the sunlight shine in for him to see, partially burning her exposed leg, before Owen hollars and diverts his attention. As Abby then awakens and attacks the Policeman in the bathroom, he reaches his hand out to Owen for help, but a horrified Owen closes the door and leaves her to feed on and kill him. Abby afterward embraces Owen softly, and he leans into her hold, before she lightly kisses him on the lips. Abby sorrowfully tells Owen that she needs to leave the town to evade being discovered and captured. Owen is devastated at the loss of her and cries looking out the window periodically, after watching her enter a cab to drive her away. (The Policeman's corpse is seen hidden in the abandoned basement room, brought there by either Abby alone or Owen assisted by Abby.)

The next night, Owen is at the school swimming pool for an after-school fitness class. As Mr. Zoric is shown to be tutoring Owen on proper swimming form and breathing, Kenny, Donald, Mark, and Jimmy start a fire outside to distract Mr. Zoric. Once Mr. Zoric is gone, the bullies lock the door to the pool from the inside, turn out the lights, and clear out the swimming pool. As Owen realizes what is happening, he runs to his locker to draw out his very small knife, which he uses in a failed attempt to defend himself against the bullies. As Owen drops the knife and flees, the bullies catch him, trag him to the deep pool, and Jimmy picks up Owen's knife to threaten him with it. Jimmy tells Owen that if he can successfully hold his breath underwater for three minutes, he will simply cut across Owen's cheek, but then tells him that if he cannot, that he will stick out one of his eyes, telling him "an eye for an ear." Owen is then held underwater by Jimmy and struggles to hold his breath. As a minute goes by, the younger bullies immediately feel guilt and the weight of the situation, and fear for Owen drowning to death. They try telling the sadistic Jimmy to stop, but he furiously refueses. As Owen is facing death by drowning, he sudenly hears strange noises happening above the water, and then suddenly the screams of the bullies. Suddenly, the pool is filled with blood, and Jimmy's decapitated body falls in. Owen pulls himself to the surface as blood and dismembered limbs of the bullies are thrown into the pool: Abby has returned to save him as earlier promised.

Sometime later, Owen is seen on a train in broad daylight, getting his ticket checked. The train attendent looks down at Owen's feet and notices a large trunk and asks if it is his. Owen replies it is and the man moves on. From the chest is tapped in Morse Code by Abby, HI [.... space ..], to which Owen gently taps back OX [--- space -..-], meaning hugs and kisses. As the film ends, the viewers are left to determine the fate of Owen and Abby for themselves.